The present invention falls within the field of electric components and is intended for use in the construction of a gas-filled over-voltage arrester which is combined, in a metal housing, with a parallel-connected voltage-limiting device, so as to form a special assembly. Such assemblies are used, in particular, wherever it is necessary to provide additional assurance against the possible failure of a gas filled over-voltage arrester.
In one known assembly of this type, the parallel-connected voltage-limiting device is developed as an air discharge gap. For example, for this purpose a thin insulating foil provided with holes is placed on one electrode of the over-voltage arrester which has a cylindrical connecting pin, the foil, in its turn, bearing an auxiliary electrode in the form of an annular disk. The air discharge gap thus formed and the over-voltage arrester are combined by means of a cylindrical metal housing to form an assembly. The housing electrically connects the auxiliary electrode to the corresponding electrode of the over-voltage arrester (U.S. Pat. No. 4,736,269). In this case, the axial clamping of the air discharge gap and the over-voltage arrester is effected by a flanged edge of the housing, which at various places on its circumference is pressed down onto the auxiliary electrode. In this known embodiment, the centering of the over-voltage arrester in the metal housing is effected by a tubular ceramic insulator of the over-voltage arrester, the outside diameter of which is greater than the outside diameter of the two electrodes of the over-voltage arrestor.
In another, similar embodiment of such an assembly, the electrodes and the insulator of the over-voltage arrester can have the same outside diameter. In that case, the centering of the over-voltage arrester in the housing, the inside diameter of which is greater than the outside diameter of the over-voltage arrester, is effected by indentations distributed uniformly on the periphery of the metal envelope of the container, the indentations being present in the region of the ceramic insulator (U.S. Pat. No. 5,142,434).
With these known assemblies, there is the danger that due to moisture the breakdown voltage of the parallel connected air discharge gap will be reduced to below the breakdown voltage of the over-voltage arrester and that thereupon the entire element will fail by a single response of the air discharge gap.
It has therefore already been proposed to encapsulate this parallel-connected air discharge gap. An arrangement for this is known in which the air discharge gap consists of a flat, stepped ceramic cylinder bearing metal layers applied to it by vapor deposition. This discharge gap is placed on the one electrode of the over-voltage arrester and soldered to it. Furthermore, a metal cap is placed over the end of the over-voltage arrester provided with the applied discharge gap, the cap being soldered on one side to a metal coating of the discharge gap and sealed on the other side with respect to the cylindrical insulator of the over-voltage arrester by means of a silver-containing epoxide resin (U.S. Pat. No. 4,707,762). Such an encapsulation of the air discharge gap is relatively expensive to produce.